Operators of the historic Angels Flight hillside railway in downtown Los Angeles had been using a tree branch to override a safety system before a derailment last month, federal safety investigators said in a report issued Thursday.

No one was hurt Sept. 5 in the latest accident on "the shortest railway in the world," though six people had to be helped from the line's two cars.

In its report, the National Transportation Safety Board said the railway's operators had broken off a branch from a nearby tree and used it to permanently depress a button so that the cars on the 298-foot track would restart during unintended stops. These stops, a sign of problems on the line, were happening regularly before the accident.

"When an unintended stop occurred, the cars would only move if the operator depressed and held the 'start' button," the report said of a safety feature "designed to prevent continued automatic movement of the cars when a fault occurs." By wedging the branch against the button, the railway's operators "negat(ed) this safety feature."

The unintended stops were happening "multiple times on each trip" for reasons unknown, though senior railway managers were aware of the use of the branch, the report said.

The report, issued on an "urgent" basis despite the near-closure of the safety board during the partial shutdown of the federal government, also questioned the effectiveness of the system's safety brake.

The railway opened in 1901 and for the price of a penny carried people between the Hill Street business district and the top of Bunker Hill.

In 2001, its two cars collided, killing one person and injuring seven others. An investigation faulted a modern gear that had replaced an original part. The emergency brake was also broken.

Rides were halted until March 2010, when the repaired and upgraded railway re-opened, ferrying tourists up and down the hill for 50 cents a ride.

In September, Angels Flight Railway President John H. Welborne said an electrical issue led to the derailment.

On Thursday, a message left at his law office was not immediately returned and no one answered the railway's publicly listed number. The recorded message said that the system was undergoing maintenance and would probably open "later in the month or in the middle of the month."

Not if the safety board has its way.

"At this time, the NTSB has not yet determined the probable cause of this accident," the board said in its report. "Nonetheless, the NTSB has identified the safety issues described above, which need to be addressed before Angels Flight returns to service to prevent a recurrence."